Rotary evaporator is mainly used for distillation and purification, but usually the rotary evaporator can only be used under negative pressure conditions, so vacuum pump should be used to achieve rapid distillation. So how to choose the vacuum pump with rotary evaporator? What should we pay attention to? EVP will lead you to understand.
There are many kinds and models of vacuum pumps, not all of them are suitable for matching with rotary evaporator. Many customers do not know whether to buy Vacuum pumps or what kind of vacuum pumps they should buy after buying rotary evaporator. In fact, you do not have to worry about this. When you buy rotary evaporator, as a professional manufacturer, we will ask you if you need vacuum pumping and matching. What type of vacuum pump is it? Of course, this depends entirely on the customer’s own needs, not compulsory purchase, if you do not need, or have a matching vacuum pump that is also possible, we as a manufacturer can provide you with some suggestions. Here’s how to choose.
Factors to Consider When Buying a Pump
How Much Vacuum Pressure Do You Need?
The solvents you’re working with and their corresponding boiling points will be the main determining factors in deciding how much vacuum pressure you need.
Depending on the boiling point of your solvent and the temperature at which you want to set your bath, you may need more or less vacuum.
For example, for a given solvent, you’ll need more vacuum when your bath is at 30°C than when it’s at 40°C. At a given bath temperature, you’ll need more vacuum for a solvent with a boiling point of 60°C than a solvent with a boiling point of 50°C.
What Pump Speed Do You Need?
The pump speed – which affects the flow rate (also referred to as evacuation rate or clearance) – is also a major concern. As mentioned, too high a speed for a given application could result in damage to the pump and other issues such as safety risks and product carryover. Aside from the practical implications, pumps with higher flow rates tend to be more expensive, and you’ll no doubt want to avoid paying more than you need to.
Flask size is an important determining factor for required pump speed. The larger the flask, the greater the air displacement required and the higher the flow rate needed. For example, a flow rate of 50 L/min may be suitable for a 20L flask whereas a rate of 70 L/min may be more suitable for a 50L flask. Also keep in mind that not all pumps are created equal. Due to the pump curves and design factors, a pump speed that is sufficient for a certain pump may not be for another pump.
If you already have a pump and are wondering if it can be used for your application, there are a couple of things to consider in terms of pump speed. Setting a low pumping speed – and as a result, low flow rate – will allow the solvent vapor more residence time in the condenser. However, many pumps are fixed-speed, so this might not be an option.
If you’re stuck with a pump that’s oversized for your application, there is an alternative option. You can reduce the flow rate of the pump by adding an additional length of vacuum hose between the condenser and the pump. The inner diameter of the hose should be smaller than that of the existing hose. The additional resistance to airflow will reduce the speed at which air is evacuated from the system. Granted, there will be some trial and error in finding the right length of hose that avoids flooding the pump but that doesn’t slow down the process too much.
When considering flow rate, you also need to consider what pressure you’ll be using your pump at. When pumps are operated at or close to their minimum pressure, the flow rate will diminish. For example, take the VP18R vacuum pump model with a flow rate of 18 L/min and a minimum pressure of 20 mbar. As that minimum pressure is approached, the pump won’t be able to clear 18 L/min.
Pump curves like the one shown above can help you determine what your flow rate will be at a given pressure. Note that not all pump curves will have the same shape.
Is Your Solvent Compatible With the Pump Materials?
With solvent vapors flowing through a pump, it’s important that the pump materials are compatible with that solvent. Ideally, a pump used in a rotovap setup should be oilless and chemical-resistant to make sure it lasts as long as possible. The most commonly used pumps for this purpose are diaphragm (or membrane displacement) pumps.
What are Your Vacuum Control Preferences?
For many rotovap applications, control over the level of vacuum is desirable to create a safer, more efficient setup. By controlling the vacuum, you can precisely adjust the pressure in the system to achieve the desired evaporation rate. This level of control can also help you avoid bumping and foaming and give you better solvent-product separation.
Some rotovaps come with a built-in vacuum controller, in which case you might not need to have a pump that has its own controller. If you do need a pump with a controller, there are analog and digital controllers to choose from, with the latter generally being pricier.